Hi Janusz, On Thu, Jul 20, 2023 at 11:35:44AM +0200, Janusz Krzysztofik wrote: > Infinite waits for completion of GPU activity have been observed in CI, > mostly inside __i915_active_wait(), triggered by igt@gem_barrier_race or > igt@perf@stress-open-close. Root cause analysis, based of ftrace dumps > generated with a lot of extra trace_printk() calls added to the code, > revealed loops of request dependencies being accidentally built, > preventing the requests from being processed, each waiting for completion > of another one's activity. > > After we substitute a new request for a last active one tracked on a > timeline, we set up a dependency of our new request to wait on completion > of current activity of that previous one. While doing that, we must take > care of keeping the old request still in memory until we use its > attributes for setting up that await dependency, or we can happen to set > up the await dependency on an unrelated request that already reuses the > memory previously allocated to the old one, already released. Combined > with perf adding consecutive kernel context remote requests to different > user context timelines, unresolvable loops of await dependencies can be > built, leading do infinite waits. > > We obtain a pointer to the previous request to wait upon when we > substitute it with a pointer to our new request in an active tracker, > e.g. in intel_timeline.last_request. In some processing paths we protect > that old request from being freed before we use it by getting a reference > to it under RCU protection, but in others, e.g. __i915_request_commit() > -> __i915_request_add_to_timeline() -> __i915_request_ensure_ordering(), > we don't. But anyway, since the requests' memory is SLAB_FAILSAFE_BY_RCU, > that RCU protection is not sufficient against reuse of memory. > > We could protect i915_request's memory from being prematurely reused by > calling its release function via call_rcu() and using rcu_read_lock() > consequently, as proposed in v1. However, that approach leads to > significant (up to 10 times) increase of SLAB utilization by i915_request > SLAB cache. Another potential approach is to take a reference to the > previous active fence. > > When updating an active fence tracker, we first lock the new fence, > substitute a pointer of the current active fence with the new one, then we > lock the substituted fence. With this approach, there is a time window > after the substitution and before the lock when the request can be > concurrently released by an interrupt handler and its memory reused, then > we may happen to lock and return a new, unrelated request. > > Always get a reference to the current active fence first, before > replacing it with a new one. Having it protected from premature release > and reuse, lock it and then replace with the new one but only if not > yet signalled via a potential concurrent interrupt nor replaced with > another one by a potential concurrent thread, otherwise retry, starting > from getting a reference to the new current one. Adjust users to not > get a reference to the previous active fence themselves and always put the > reference got by __i915_active_fence_set() when no longer needed. > > v3: Fix lockdep splat reports and other issues caused by incorrect use of > try_cmpxchg() (use (cmpxchg() != prev) instead) > v2: Protect request's memory by getting a reference to it in favor of > delegating its release to call_rcu() (Chris) > > Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/8211 > Fixes: df9f85d8582e ("drm/i915: Serialise i915_active_fence_set() with itself") > Suggested-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> > Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@xxxxxxxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> # v5.6+ thanks for the offline clarification on this! It's another good catch of yours :) Reviewed-by: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx> Thank you! Andi
